The present invention generally relates to semiconductor devices and more particularly to a non-linear optical device that has an improved rate for recovery after causing transition.
The U.S. patent application Ser. No. 07/404,958, now abandoned, and corresponding European patent application No.89 116 667.0 describe a non-linear optical device that changes its transparency upon incidence of an optical beam. In such a device, a two-dimensional quantum well is formed for creating excitons that absorb the incident optical beam. Upon incidence of the optical beam, the excitons decompose into electrons and holes. Electrons and holes may be excited simultaneously. Thereby the refractive index of the device changes. Such a change is also accompanied with the change of transparency of the device.
In such a non-linear optical device, one can control the optical transmittance by injecting a control optical beam that interacts with the excitons in the two-dimensional quantum well. In response to the irradiation of a strong control optical beam, the quantum well is saturated by the electrons and holes that are created by the optical absorption or decomposition of the excitons, and the device loses its ability of optical absorption. In other words, the device changes from an opaque state to a transparent state. On the other hand, the recovery to the original opaque state after the interruption of the control optical beam generally needs much longer time because of the time needed for the electrons and holes accumulated in the quantum well to dissipate.
In order to achieve the quick recovery of the original state upon the interruption of the incident optical beam, the non-linear optical device disclosed in the above references uses the so-called TBQ (tunneling bi-quantum well) structure that facilitates the quick escape of the electrons from the quantum well of the device.
FIG. 1 shows the band structure of the TBQ device. Referring to FIG. 1, the TBQ structure includes an active layer 34 confined two-dimensionally by a pair of barrier layers 33 and 35 that have a large band gap, wherein the barrier layer 35 is located above the active layer 34 while the barrier layer 33 is located below the active layer 34. Thereby, there are formed a quantum level E1 for the electrons and a quantum level H1 for the holes in the active layer 34. The excitons have a level close to but slightly lower than the quantum level E1 because of the Coulomb interaction acting between the electron and the hole that form the exciton. The barrier layers 33 and 35 are formed from a material having a large band gap as already described and forms the potential barrier that in turn defines the two-dimensional quantum well.
Above the barrier layer 35, there is provided a layer 36 that forms anther two-dimensional quantum well. Similarly, below the barrier layer 33, there is provided a layer 32 that forms still another two-dimensional quantum well. It should be noted that the layer 36 is bounded by the barrier layer 35 and another barrier layer 37 that has a large band gap similar to the barrier layer 35. Similarly, the layer 32 is bounded by the barrier layer 33 and another barrier layer 31 that has a large band gap similar to the barrier layer 33.
Here, the width of the layer 32 or 36 is set substantially larger than the layer 34 such that there is formed a quantum level E1' of electrons at a level substantially lower than the quantum level E1. Associated therewith, a quantum level H1' of holes is formed at a level substantially lower than the quantum level H1 in terms of the energy of the holes. Similarly, the width of the layer 32 is set substantially larger than the layer 34 such that there are formed a quantum level of electrons that is identical with the quantum level E1' and a quantum level of holes that is identical with the quantum level H1'.
In the foregoing TBQ structure, the thickness of the barrier layers 33 and 35 is set substantially small such that the electrons at the quantum level E1 of the quantum well layer 34 can escape freely to the quantum level E1' of the quantum well layer 36 by tunneling through the barrier layers 33 and 35. More specifically, when the electrons are excited in the layer 34 from the quantum level H1 to the quantum level E1 in response to the absorption of the incident optical beam or the control optical beam, the electrons immediately escape to the layers 32 and 36 by tunneling through the barrier layers 33 and 35. Thereby, the problem of unwanted residence of the electrons in the layer 34 after the incident optical beam is interrupted is eliminated, and the recovery time of the non-linear optical device can be significantly improved.
In such a conventional TBQ device, however, there exists a problem in that not only the quantum well layer 34 but also the quantum well layers 32 and 36 cause the optical absorption. It should be noted that the energy needed for causing the transition of electrons from the quantum level H1' to the quantum level E1' is much smaller than the energy needed for causing the transition of the electrons from the quantum level H1 to the quantum level E1. In other words, the incidence of an optical beam that has an energy sufficient to cause the transition of electrons in the quantum well layer 34 inevitably causes the transition of electrons in the quantum well layers 32 and 36, and the transition of the electrons in the quantum well layers 32 and 36 causes an unwanted absorption of the optical beam. In other words, the conventional device of FIG. 1 has suffered from the problem of low S/N ratio.
The device of FIG. 1 has another problem of the residence of the holes in the quantum well layer 34 after the interruption of the control optical beam. It should be noted that the holes have an effective mass that is much larger than the effective mass of the electrons. Thereby, the probability of tunneling of the holes through the barrier layers 33 and 35 becomes much smaller than the probability of tunneling of the electrons, and there is a tendency that the expected quick recovery of the optical property is not obtained due to the residence of the holes in the layer 34 even after the interruption of the optical beam.
Such an unwanted residence of the holes in the layer 34 causes another problem of the shift in the energy level of the layer 34 with respect to the energy level of the other layers. More specifically, the energy level of the layer 34 tends to be lowered in the band diagram with respect to the layers 32 and 36 when there are holes accumulated in the layer 34. Ultimately, there may be a case wherein the energy level E1 of the layer 34 may become equal to or close to the energy level E1' of the layers 32 and 36. When this occurs, no efficient removal of the electrons from the layer 34 to the layers 32 and 36 through the barrier layers 33 and 35 is expected.
In the structure of FIG. 1, the unwanted residence of the holes in the active layer 34 may be prevented by reducing the thickness of the barrier layers 33 and 34 such that the tunneling probability of the holes increases. When such an approach is adopted, however, there arises a problem of excessive escaping of the electrons from the layer 34 to the layers 32 and 36 because of the smaller effective mass of electrons. Thereby, the formation of excitons in the active layer 34 may not be obtained due to the depletion of the electrons in the layer 34. In the structure of FIG. 1, it should be noted that both the electrons and holes exit from the active layer 34 through the barrier layers 33 and 35.